photovoltaic 101

ritelec

Senior Member
Anyone have any suggestions for photovoltaic installations learning.....readings... cliff notes... I know it's in the book (and maybe what I'm looking for as now I've seen little today)but looking for simple one line diagrams and layouts..
Basic protection and layout out.. and design

I've never needed to know it (and kinda still don't) but there is a company installing a system for the church. They are putting pv panels and inverters on/in one building then feeding that building and two others from those panels and inverters in that one building..

The mechanic was kind enough to walk through some things which I found interesting...


They will power the inverter from the building they want to supply, and the inverter uses those conductors to back feed that building..... WOW...

My questioning would be as far as them tapping from each buildings mains (length of taps and ocp), labelling ..., disconnect locations (as it seems that they may be entering the building below grade and its questionable if they should install a disconnect before entering the building...

Seems like they are putting inverters together to handle the loads and he will be paralleling smaller conductors between them and I'm curious in his design should have these conductors protected...

How the wires (dc) are protect (ocp) as they attach to the pv panel (is array the word) and go the the inverter???

I will see more as the next couple of weeks go by, but would like to know for my own head and to also see that it is kinda right..
 

jaggedben

Senior Member
I didn't learn from any books. I took a couple classes and then started working in it. I think Dunlop's book is considered a kinda classic but it's probably pretty out of date. You're probably better off using Google to find a most recent publication. Something that covers microinverters and optimizers which are prevalent these days.

As far as overcurrent protection, PV power sources can't really overload their own circuits. So protection, when it's required, is there against parallel sources. On the DC side that would be other PV panels or sometimes batteries. On the AC side it's the grid. On the AC side the utility is usually going to provide essentially all the fault current, so generally an inverter is not wired any different than a load. Where you connect it matters though, see article 705.
 

ritelec

Senior Member
was thinking and watching some Mike Holt videos on you tube...
he talks about several different types of systems and in one type a breaker from the load side of the main feeds the pv system... he talks about oc protection and possible doubling up of current on feeds to sub panels....
has me thinking of the system this company is putting in........
From what I understand of it they are tapping onto the LINE side of the service.
building 1 is a ct cab 3 phase that has a line/load meter on it (no ct's) and it feeds a 225 amp main breaker....
he will be tapping onto the line before the main breaker.
building 2 is a ct cabinet with ct's he will not touch the cabinet but will tap on before the main breaker enclosure...
building 3 is a house... guessing he's tapping on before the main breaker...
I'm guessing his disconnects at the tap point are going to be breakers or fused disconnects??


In all those cases, I'm guessing the main breakers will protect the existing service wire to the main breaker.. but was wondering theory..

the utility power feeds the service main, it will be tapped to feed the inverter, power comes back on those same conductors to the existing utility wiring feeding the same main breaker.

When the system is on, how does the building know which power source to take power from? The utility or the PV..... they are the same wire with two different power sources on it...

WOW
 

kwired

Electron manager
PV101, light collected by the array is converted to electrical energy. End of class.

You maybe looking for PV 102 and 103?:D
 

ggunn

PE (Electrical), NABCEP certified
was thinking and watching some Mike Holt videos on you tube...
he talks about several different types of systems and in one type a breaker from the load side of the main feeds the pv system... he talks about oc protection and possible doubling up of current on feeds to sub panels....
has me thinking of the system this company is putting in........
From what I understand of it they are tapping onto the LINE side of the service.
building 1 is a ct cab 3 phase that has a line/load meter on it (no ct's) and it feeds a 225 amp main breaker....
he will be tapping onto the line before the main breaker.
building 2 is a ct cabinet with ct's he will not touch the cabinet but will tap on before the main breaker enclosure...
building 3 is a house... guessing he's tapping on before the main breaker...
I'm guessing his disconnects at the tap point are going to be breakers or fused disconnects??


In all those cases, I'm guessing the main breakers will protect the existing service wire to the main breaker.. but was wondering theory..

the utility power feeds the service main, it will be tapped to feed the inverter, power comes back on those same conductors to the existing utility wiring feeding the same main breaker.

When the system is on, how does the building know which power source to take power from? The utility or the PV..... they are the same wire with two different power sources on it...

WOW
It's all in 705.12(B) in the 2017 NEC and in 705.12(D) in earlier editions. Read and understand it. I agree with kwired, though; it's a little above the basics.
 

ritelec

Senior Member
It's all in 705.12(B) in the 2017 NEC and in 705.12(D) in earlier editions. Read and understand it. I agree with kwired, though; it's a little above the basics.
Read through the 2014....... evelyn wood....... suggested reading and 685, 690

picked up some of what I was asking, will look into it deeper..... BUT, nowhere did I see the theory of how two power sources use the same conductors to the main and the main magically knows which one to use ???
 

romex jockey

Senior Member
BUT, nowhere did I see the theory of how two power sources use the same conductors to the main and the main magically knows which one to use ???
this ^^^

iI follow a few solar guys around, hook 'em up . I'm always asking here, it usually ends in a 705 ref

but it doesn't help me understand it all at an atomic level...or sub-atomic....?

I'm a simple spark....not a physicist.....


~RJ~
 

ggunn

PE (Electrical), NABCEP certified
this ^^^

iI follow a few solar guys around, hook 'em up . I'm always asking here, it usually ends in a 705 ref

but it doesn't help me understand it all at an atomic level...or sub-atomic....?

I'm a simple spark....not a physicist.....


~RJ~
It's a parallel circuit of sources and loads, but the sources are a current source (the PV inverter) in parallel with a voltage source (the grid). This is what gives rise to the notion that your loads "preferentially" receive your PV output over being serviced by the grid. The PV pushes out whatever it can irrespective of the local loading and the grid either makes up the difference or takes the surplus.
 

Carultch

Senior Member
this ^^^

iI follow a few solar guys around, hook 'em up . I'm always asking here, it usually ends in a 705 ref

but it doesn't help me understand it all at an atomic level...or sub-atomic....?

I'm a simple spark....not a physicist.....


~RJ~
You never really know what load draws power from what source. The energy just flows "downhill". The sources set up voltages at the point where connected to the circuit, and then enough current will flow out of each one, in order that Ohm's and Kirchhoff's laws will be satisfied. If you can understand the microscopic views of Ohm's and Kirchhoff's laws, you can understand how a grid of conductors distributes power and "knows" how much to take from each source.

Consider this example. R3 represents the total loads. If we were dealing with on-site loads and neighborhood grid loads, we'd connect another resistor in parallel with R3 to represent the neighborhood grid loads, and put a meter between the two, representing your service meter. R1 and R2 represent the wire resistance from both sources. V1 and V2 represent the two sources themselves, and the output voltages. You can think of them as batteries for a simple example, but the concept can also extend to AC sources as well (generators, utility services, inverters, etc). The currents must balance at every point, thus I3 = I1 + I2. The voltage across the load due to each source, will also have to be the same. So if V1 and V2 are not equal, the voltage drops across R1 and R2 will be as much as necessary, until they are equal. The current through each resistor will be what is needed to make this so.

This means that the currents, I1 and I2, will be what is needed to satisfy the following conditions:
V1 - I1*R1 = V2 - I2*R2
I3 = I1 + I2
I3*R3 = (V1 - I1*R1)

R1, R2, R3, V1, and V2 are given.




In the case of an inverter, there is a feedback loop that continuously monitors grid voltage, and generates a slighty higher voltage to "push out" whatever power it can. It functions as a current source, rather than a voltage source, because its output voltage is flexible to adapt to the grid.
 

K8MHZ

Senior Member
Think in terms of an automobile. It has two sources of energy, the battery and the alternator, connected in parallel with the loads. The battery voltage is 12.6 at rest, the alternator is around 14 while running. When the alternator is putting out 14 volts, it is supplying energy to all the loads, including the battery. When the alternator shuts off or dies, the battery will be the source of energy and will no longer be a load. The other loads don't 'care' where the energy comes from, they just use it.
 

ggunn

PE (Electrical), NABCEP certified
In the case of an inverter, there is a feedback loop that continuously monitors grid voltage, and generates a slighty higher voltage to "push out" whatever power it can. It functions as a current source, rather than a voltage source, because its output voltage is flexible to adapt to the grid.
That's simplified, of course. A grid tied PV inverter is a current source because PV modules are current sources. It doesn't need to raise the voltage to "push out" current; it could just as easily deliver current into a superconductor where there would be no voltage drop.
 

GoldDigger

Moderator
Staff member
That's simplified, of course. A grid tied PV inverter is a current source because PV modules are current sources. It doesn't need to raise the voltage to "push out" current; it could just as easily deliver current into a superconductor where there would be no voltage drop.
Speaking of oversimplification....
A PV module is by its nature a current source.
A grid interactive inverter is best characterized as a current source because it is designed that way, and would be whatever provided its input power.

Hybrid inverters in grid interactive mode are current sources even when running off battery and the SMA Secure Power Supply is a voltage source even though it is powered by PV.

Sent from my XT1585 using Tapatalk
 

romex jockey

Senior Member
I appreciate the responses fellas , thankfully there's no law against educating a redneck spark :)

But here's one that turns my head around ......

the PV guys come in and do their thing, then determine the panel is too small, by too small i mean the buss

they'll have me place a 200A MLO into a 100A (meter/100A main) system for this

So we end up with a 200A MLO off a 100A service , i just can't 'get' why the focus is on the panel being inadequate when the the rest of the serive is ok?

~R (brain fried in vermont)J~
 

kwired

Electron manager
I appreciate the responses fellas , thankfully there's no law against educating a redneck spark :)

But here's one that turns my head around ......

the PV guys come in and do their thing, then determine the panel is too small, by too small i mean the buss

they'll have me place a 200A MLO into a 100A (meter/100A main) system for this

So we end up with a 200A MLO off a 100A service , i just can't 'get' why the focus is on the panel being inadequate when the the rest of the serive is ok?

~R (brain fried in vermont)J~
How often it is likely to happen? IDK, on a dwelling seems not so likely, but I think they are afraid of drawing 100 amps from the utility plus whatever the PV is able to deliver - so that 100 amp bus is capable of having more than 100 amp on it without tripping anything.
 

ggunn

PE (Electrical), NABCEP certified
Speaking of oversimplification....
A PV module is by its nature a current source.
A grid interactive inverter is best characterized as a current source because it is designed that way, and would be whatever provided its input power.

Hybrid inverters in grid interactive mode are current sources even when running off battery and the SMA Secure Power Supply is a voltage source even though it is powered by PV.

Sent from my XT1585 using Tapatalk
Well, OK, but a PV module is only mostly a current source. When V gets to be between Vmp and Voc, I comes down. Inverters powered by PV modules can behave as voltage sources by tweaking MPPT but only in that very narrow slice of the DC voltage between Vmp and Voc. It seems to me that making a battery inverter behave as a current source would be easier than the other way round.

Ideal voltage and current sources do not exist in the real world, just like ideal resistors, capacitors, and inductors. The devil is in the details. But this is beyond the scope of PV 101, innit? :D
 
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